Field
This invention relates generally to a feed mechanism for feeding a filament through a print head in a fused deposition modeling (FDM) printer and, more particularly, to a feed mechanism for feeding a filament into a nozzle in a print head of an FDM printer, where the feed mechanism includes two counter-rotating tank tread type drive elements.
Discussion
Fused deposition modeling (FDM) is an additive manufacturing (AM) technology and is a technique used for 3D printing. More specifically, FDM is a process where a material is laid down layer by layer from a spool of material that is fed into a heated nozzle as a filament to build a desired product. The material from the spool is fed into the heated nozzle, which causes the material to become molten, where the molten material immediately begins to harden once it is extruded from the nozzle. A typical filament for these types of printers will have a diameter of 1.75 mm or 3 mm. FDM utilizes software that processes a stereolithography file (STL) file and includes mathematically slicing and orienting the model to build up the product layer by layer. Support structures may also be built layer by layer using a second spool of material, for example, a soluble material may be provided to build a soluble support structure.
Various materials may be used for FDM, such as acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), polycarbonate (PC), polyamide (PA), polystyrene (PS), lignin, rubber, carbon fibers, thermoplastics, polyphenylsufone, ultra high molecular weight polyethylene (UHMWPE) Dyneema, Ultem 9085, high impact polystyrene (HIPS), nylon, high density polyethylene (HDPE) eutectic materials, plasticine, room temperature vulcanization (RTV) silicone, etc. Typically the material is selected to increase the structural properties of the printed products that are made therefrom.
Some printing filaments have additional materials added to provide different electrical, mechanical and/or cosmetic properties. Some of these materials can dramatically increase the thermal conductivity of the filament. During the printing process, the hot end of the printer will heat the filament, where heat will move through the filament towards the source of the filament. For filaments with a high thermal conductivity, this heat transfer can cause the filament entering the printer to approach its glass transition temperature, which softens the filament and reduces the effectiveness of the feed mechanism.
A feed mechanism is employed in a print head assembly of the FDM printer that feeds the filament to the nozzle from which it is extruded. The distance between the feed mechanism and the nozzle varies from printer to printer, and can be, for example, in the range of 75 mm-1 m. It is critical that the feed mechanism feeds the filament into the nozzle at a constant rate so that the heated material is properly laid down to form the product. The feed rate is generally a function of the temperature of the nozzle and the force applied by the feed mechanism to the filament. The ability of the feed mechanism to properly feed the filament to the nozzle is also often highly reliant on the type of material being fed. For some materials it is not uncommon that the filament gets jammed within the feed mechanism and is prevented from being provided to the nozzle, possibly a result of the nozzle being at a too low of temperature or the filament buckling between the feed mechanism and the nozzle. Further, heat can be transferred from the nozzle up the filament as it is being fed, possibly causing the filament to approach its glass transition temperature where it begins to melt prior to it reaching the nozzle, which could plug up the nozzle. Typically, what happens is that a feed failure causes the filament to break or be prevented from being extruded by the nozzle, where the machine does not detect this failure and continues with its print routine.
A typical feed mechanism for an FDM printer employs a roller bearing and a drive gear, where the drive gear drives the filament by applying pressure against the roller bearing. More specifically, the roller bearing provides a radial force on the filament as it is being driven by the drive gear. It is believed that this type of feed mechanism is a major contributing factor to feed failures because the force on the filament that drives it to the nozzle is a point source at the location where the drive gear contacts the filament, which is not at the center of the filament, and which causes a bending moment to be applied to the filament. Further, the contact pressure of the drive gear on the filament sometimes operates to remove material from the filament, thus reducing the contact force on the filament that drives the filament into the nozzle.